Sonobuoys and related seismic sensing devices have been in use for some time to sense acoustic phenomena of interest. A common design suspends a hydrophone from a spar buoy like float which also supports a transmitter-antenna combination. Signals representative of impinging acoustic energy on the hydrophone are transmitted to a distant monitoring station for appropriate data processing. This design provided acceptable performance in the audiofrequency range under calm conditions. An example of this type of conventional system is disclosed in the U.S. Pat. No. 3,982,222 entitled "Deep Hydrophone String" and issued on Sept. 21, 1976 to Robert J. Urich. Although this design calls for the lower end of the array being achored, fluctuations from the buoyant member will induce accelerations and pressure variations at the individual hydrophone elements creating what might be an unacceptable signal-to-noise ratio. The objectionable noise is the result of mechanical disturbances being transmitted to the hydrophone elements which, in turn, create representative spurious signals to be amplified. The objected-to disturbances are largely the product of the buoyant member's reacting to the effects of current, wind and surface wave action. These disturbances cannot be anticipated to allow their being compensated for in a sophisticated data processing technique, since they are extremely variable. For example, with the wind, waves and current all going the same way a certain pattern of mechanical disturbances is created. However, when the current is perpendicular or at some angle to the direction of the waves and the wind is from still another direction, a rigidly suspended hydrophone element will produce totally dissimilar spurious signals.
The self-propelled array system disclosed in U.S. Pat. No. 4,004,265 issued to Thomas E. Woodruff, et al on Jan. 18, 1977 seeks to avoid the generation of spurious signals due to the interaction of a transmitter float. Their plan is to sink the array and anchor it on the bottom while relying on other means for deploying it in a more or less horizontal plane. However, certain places in the ocean are much too deep for such a buoying scheme or tactical expediency may dictate that a float supported array is the better choice.
The subsurface system of U.S. Pat. No. 3,818,523 issued to Steven L. Stillman, Jr. on June 25, 1974 is another noteworthy attempt to provide for increased reliability. This moored system actually takes advantage of the ocean's currents to submerge or refloat the sonobuoy package. In one modification a portion of his tethering cable is made buoyant. This must necessarily reduce the transmission of hydromechanical disturbances to the hydrophone elements although it appears that the purpose of such a modification is to get the cable off the ocean's floor.
Thus, from the foregoing it is apparent that there currently exists in the state of the art a continuing need for an unmoored hydrophone arrangement which hydromechanically decouples a suspended hydrophone arrangement from the effects of current, wind and surface wave action.